The mass number, also known as the atomic mass, is usually located below the element symbol on the periodic table.
Understanding Mass Number
The mass number represents the average mass of all an element's isotopes. Each isotope of an element has a different number of neutrons, causing a variation in mass. Because different isotopes exist in different proportions in nature, we must calculate the average mass based on each isotope's natural abundance.
Location on the Periodic Table
On the periodic table:
- The element's symbol is usually located in the center of the cell.
- The atomic number is normally found above the symbol.
- According to the provided reference, the mass number (atomic mass) is typically located below the element symbol.
Calculating Average Mass
The atomic mass is not a whole number because it is a weighted average:
- Each isotope's mass is multiplied by its fractional abundance in nature.
- These values are then added together to get the average atomic mass.
Example
| Isotope | Mass (amu) | Abundance (%) | Mass * Abundance (amu) |
|---|---|---|---|
| Isotope 1 | 1.0078 | 99.985 | 1.0077 |
| Isotope 2 | 2.0141 | 0.015 | 0.0003 |
| Total | 1.008 |
- This example shows that the average atomic mass would be 1.008 based on the isotopes listed, which would typically be displayed on the periodic table below the element symbol for Hydrogen (H).
Key Takeaways
- Location: The mass number is found below the element symbol on the periodic table.
- Average Mass: The number represents the average mass of all isotopes of that element.
- Isotopes: The average mass takes into account the natural abundance of each isotope.
